1. Field of the Invention
The present invention relates to a chromatic dispersion distribution measurement apparatus for measuring a chromatic dispersion distribution in an optical device to be measured, such as an optical fiber, and a method for the same.
2. Description of Related Art
In recent years, in order to satisfy the demands for high speed information communications, optical communication systems using optical fibers have been constructed. One of the factors in preventing the high speed signal transmission and the long transmission distance in the above optical communication systems, is the chromatic dispersion. The chromatic dispersion is a phenomenon caused by varying the speeds of lights transmitted in a medium, with the wavelength of the light. In the construction of the optical communication systems, it is necessary to grasp the chromatic dispersion characteristic in detail.
A chromatic dispersion distribution measurement apparatus for measuring the chromatic dispersion is shown in, for example, Japanese Patent Application Publication No. Tokukai-Hei 10-83006 (corresponding to the U.S. Pat. No. 5,956,131 and the European Patent Application No. 0819926A2). In the publication, the chromatic dispersion distribution measurement apparatus measures the dispersion distribution in a longitudinal direction of a fiber to be measured, as follows. Two lights having different wavelengths from each other are inputted into the fiber to be measured. A specific wavelength component is extracted by an optical bandpass filter from a four-wave mixed light caused by the interaction between these two lights. A light having the extracted specific wavelength component is inputted into an Optical Time Domain Reflectometer (OTDR).
FIG. 3 shows the relationship between the transmission distance along the fiber (abscissa axis) and the intensity of the back-scattered light generated at any portion of the fiber to be measured (ordinate axis) when the two lights having different wavelengths from each other are launched into the fiber to be measured. This intensity distribution (shown in solid line) is observed by the OTDR. As illustrated in this figure, the intensity changes periodically according to the transmission distance. Also, the intensity decreases as the transmission distance becomes longer. Hereinafter, the intensity distribution shown in FIG. 3 is frequently called an OTDR waveform.
In such a chromatic dispersion distribution measurement apparatus, in order to obtain the chromatic dispersion distribution in the fiber to be measured, the Hilbert transform is performed on the OTDR waveform data observed by the OTDR. However, as shown in FIG. 3, since the OTDR waveform (intensity) fluctuates sharply at a near-end of an optical fiber (around a zero distance), the following problem is caused. There is an area (near-end dead zone) in which the chromatic dispersion distribution is not exactly obtained because of the limit of the Hilbert transform. That is, there is a dead zone around the zero distance where the Hilbert transform cannot be performed exactly.
In order to solve the above-described problems, an object of the present invention is to provide a chromatic dispersion distribution measurement apparatus and a method for the same, for controlling a power of an input light to be input into the optical device to be measured, for example, so as to reduce the near-end dead zone, that is, the effect of the sharpness of the intensity around the zero distance in the light intensity distribution.
That is, in accordance with the first aspect of the present invention, a chromatic dispersion distribution measurement apparatus, comprises:
an input light power calculation unit for calculating a power value of an input light to be input into an optical device to be measured, in accordance with a previously measured power of an output light from the optical device to be measured as a function of a transmission distance along the optical device to be measured;
an input light control unit for controlling an input light power of the input light to the optical device to be measured in accordance with the power value of the input light, which is calculated by the input light power calculation unit;
a scattering light power measuring unit for measuring a scattered light power of a back-scattered light scattered from each portion of the optical device to be measured, as a function of the transmission distance, when the input light of which the input light power is controlled by the input light control unit, is input into the optical device to be measured; and
a chromatic dispersion distribution calculation unit for calculating a chromatic dispersion distribution value in the optical device to be measured in accordance with the scattered light power which is measured by the scattering light power measuring unit.
In accordance with the second aspect of the present invention, a chromatic dispersion distribution measurement method, comprises:
calculating a power value of an input light to be input into an optical device to be measured, in accordance with a previously measured power of an output light from the optical device to be measured as a function of a transmission distance along the optical device to be measured;
controlling an input light power of the input light to the optical device to be measured in accordance with the calculated power value of the input light;
measuring a scattered light power of a back-scattered light scattered from each portion of the optical device to be measured, as a function of the transmission distance, when the input light of which the input light power is controlled in the controlling, is input into the optical device to be measured; and
calculating a chromatic dispersion distribution value in the optical device to be measured in accordance with the measured scattered light power.
According to the first and the second aspects of the present invention, by controlling the input light power of the input light to the optical device to be measured in accordance with the previously measured power of an output light from the optical device to be measured as a function of a transmission distance along the optical device to be measured, it is possible to give a correct measurement of the chromatic dispersion distribution.
The input light power calculation unit may vary the power value of the input light to the optical device to be measured, according to a level of the previously measured power of the output light from the optical device to be measured.
The input light power calculating may be carried out by varying the power value of the input light to the optical device to be measured, according to a level of the previously measured power of the output light from the optical device to be measured.
Therefore, by varying the power value of the input light to the optical device to be measured, according to a level of the previously measured power of the output light from the optical device to be measured, it is possible to increase the reliability of the measurement of the chromatic dispersion distribution.
The chromatic dispersion distribution measurement apparatus may further comprise a calculated data combination unit for combining a first chromatic dispersion distribution data which is calculated by the chromatic dispersion distribution calculation unit when the input light power is small, with a second chromatic dispersion distribution data which is calculated by the chromatic dispersion distribution calculation unit when the input light power is large.
The chromatic dispersion distribution measurement method may further comprise combining a first chromatic dispersion distribution data which is calculated when the input light power is small, with a second chromatic dispersion distribution data which is calculated when the input light power is large.
Therefore, by combining each chromatic dispersion distribution data calculated on each input light power, it is possible to increase the reliability of the measurement of the chromatic dispersion distribution further.
In accordance with the third aspect of the present invention, a chromatic dispersion distribution measurement apparatus, comprises:
a power calculation unit for calculating a power value of an input light to be input into an optical device to be measured, in order to calculate a chromatic dispersion distribution in the optical device to be measured, in accordance with an output light power of an output light from the optical device to be measured as a function of a transmission distance along the optical device to be measured; and
a control unit for controlling an input light power of the input light to the optical device to be measured in accordance with the power value of the input light, which is calculated by the power calculation unit.
The power calculation unit may determine the power value of the input light to be approximately half of the output light power of the output light from a near-end portion of the optical device to be measured, when the output light power of the output light from the near-end portion of the optical device to be measured is too large to calculate the chromatic dispersion distribution.
The power calculation unit may calculate each power value of the input light in accordance with each output light power of the output lights outputted from a plurality of points of the optical device to be measured; and
the control unit may control each input light power of the input light in accordance with each power value of the input light, which is calculated by the power calculation unit.
The chromatic dispersion distribution measurement apparatus may further comprise: a measuring unit for measuring each scattered light power of a light scattered from the optical device to be measured, on each input light power controlled by the control unit;
a chromatic dispersion calculation unit for calculating each chromatic dispersion distribution in the optical device to be measured, on each scattered light power measured by the measuring unit; and
a combination unit for combining a part of one chromatic dispersion distribution calculated on one input light power, with a part of another chromatic dispersion distribution calculated on another input light power.
In accordance with the fourth aspect of the present invention, a chromatic dispersion distribution measurement method, comprises:
calculating a power value of an input light to be input into an optical device to be measured, in order to calculate a chromatic dispersion distribution in the optical device to be measured, in accordance with an output light power of an output light from the optical device to be measured as a function of a transmission distance along the optical device to be measured; and
controlling an input light power of the input light to the optical device to be measured in accordance with the calculated power value of the input light.
The calculating may be carried out by determining the power value of the input light to be approximately half of the output light power of the output light from a near-end portion of the optical device to be measured, when the output light power of the output light from the near-end portion of the optical device to be measured is too large to calculate the chromatic dispersion distribution.
The calculating may be carried out by calculating each power value of the input light in accordance with each output light power of the output lights outputted from a plurality of points of the optical device to be measured; and
the controlling may be carried out by controlling each input light power of the input light in accordance with each calculated power value of the input light.
The chromatic dispersion distribution measurement method may further comprise: measuring each scattered light power of a light scattered from the optical device to be measured, on each controlled input light power;
calculating each chromatic dispersion distribution in the optical device to be measured, on each of the measured scattered light power; and
combining a part of one chromatic dispersion distribution calculated on one input light power, with a part of another chromatic dispersion distribution calculated on another input light power.